Grid oscillation analysis method and apparatus therefor

ABSTRACT

A method for determining the contribution of a grid subsystem ( 12 ) to oscillations in grid frequency experienced by an external electrical grid ( 14 ) in an electrical power network ( 10 ). A measurement of grid frequency is taken in the grid sub-system ( 12 ) and/or the external electrical grid ( 14 ). The measurement(s) of grid frequency can be used to extract oscillations in grid frequency in the grid subsystem ( 12 ) and/or the external electrical grid ( 14 ). A measurement of active power is recorded on a transmission line ( 16   c ) between the grid subsystem ( 12 ) and the external electrical grid ( 14 ). The phase relationship between the oscillations in grid frequency and the oscillations in active power allows the contribution of the grid subsystem ( 2 ) to oscillations in grid frequency in the external electrical grid ( 14 ) to be determined.

FIELD OF THE INVENTION

The present invention relates to a grid oscillation analysis method andapparatus therefor. More specifically, the invention relates to a methodof determining whether a grid subsystem in an electrical grid iscontributing to oscillations in grid frequency.

BACKGROUND TO THE INVENTION

Electric power networks are prone to grid oscillations. For exampleoscillations may be seen in grid frequency, grid power, grid voltageangle and/or the speed of generators and other equipment used as part ofa grid.

The disclosure herein refers principally to oscillations in gridfrequency and oscillations in grid power. However, it should be notedthat oscillations of other grid metrics are intentionally encompassedwithin the scope of the disclosure.

Oscillations in grid frequency can be prejudicial to the properoperation of an electrical grid. Oscillations in grid frequency may, forexample, cause interconnector tripping, relay tripping, loss of load orgenerating plant stress. Such oscillations often involve (and affect)many generating plants. For example, frequency oscillations may becaused by rotor speed oscillations in generators. Also, suchoscillations in grid frequency may span boundaries between network areasunder the control of different operating companies or perhaps evennational boundaries.

For the avoidance of doubt, it is noted that as used herein the term“oscillation in grid frequency” when relating to electrical powernetworks encompasses variations in a nominal frequency of the network.For example, an electrical power network may provide power at afrequency of 60 Hz. However, the frequency may vary between say 59 Hzand 61 Hz with a period of say 20 seconds, although this is an extremeexample of oscillations in grid frequency. Therefore, the term“oscillation in grid frequency” refers to the periodic deviations overtime of the frequency in an electrical power network, e.g. the periodicswings of the nominal 60 Hz grid frequency between 59 Hz and 61 Hz.

The variations in grid frequency and/or power also have an oscillationfrequency associated with them.

The term “grid frequency” encompasses the frequency of power provided bythe network, e.g. the nominal 50 Hz or 60 Hz frequency of the grid andany deviation from the nominal value.

Under certain network conditions, the oscillations in grid frequency maybe poorly damped or even unstable. FIG. 1 illustrates an unstableoscillation in grid frequency that led to system separation andload-shed relay tripping. More specifically, FIG. 1 shows a graph ofgrid frequency over time, in which a nominal 60 Hz system oscillates atlow frequency with a period of about 20 seconds, with swings from 59 Hzto 61 Hz.

The present inventors have appreciated that it is at present difficultto determine if a particular grid subsystem, such as a generating plant,control area or some defined region of the grid is contributingpositively or negatively to the stability of a oscillation in gridfrequency, or merely responding to the oscillation in grid frequency.

It is therefore an object for the present invention to provide anelectrical grid comprising apparatus that is operable to determine if agrid subsystem is contributing positively or negatively to the stabilityof oscillation in grid frequency.

It is a further object for the present invention to provide a method ofdetermining if a grid subsystem in an electrical grid is contributingpositively or negatively to the stability of oscillations in gridfrequency.

STATEMENT OF INVENTION

According to the disclosure in a first aspect there is provided a methodof determining the contribution of a grid subsystem to oscillations ingrid frequency in an external electrical grid in an electrical powernetwork, the method comprising: receiving a first quantity correspondingto the oscillations in grid frequency in the external electrical grid;receiving at least one second quantity, the at least one second quantitycorresponding to oscillations in active power conveyed on at least oneelectrical connection between the grid subsystem and the externalelectrical grid; and determining at least one phase relationship betweenthe first quantity and the at least one second quantity to determine thecomponent of the second quantity that contributes to oscillations ingrid frequency in the external electrical grid.

The applicant has identified that by determining a phase relationshipbetween oscillations in grid frequency and oscillations in the activepower conveyed on an electrical connection, it can be determined whetherthe subsystem is contributing to the oscillation or responding to it.Further, the magnitude of the component of oscillations in power inquadrature with the oscillations in grid frequency allows thequantification of the contribution of the grid subsystem to theoscillations in grid frequency.

As used herein, the term “grid subsystem” encompasses one or moreelectrical elements that may form part of an electrical power network.For example, a grid subsystem may comprise one or more generators andsubstations.

As used herein, the term “external electrical grid” encompasses anyelements of an electrical power network that are external to a gridsubsystem. The external grid may therefore represent the wider grid or,put another way, the remainder of the electrical power network.

As used herein, the term “electrical power network” encompasses anynetwork of electrical elements that are interconnected for the purposeof providing electrical power.

The first quantity may be measured in the grid subsystem. Alternatively,the first quantity may be measured in the external electrical grid.

It is noted that a phase relationship is a relationship between twoquantities oscillating at the same frequency, such that there is aconsistent time shift between the peaks of the oscillating quantities.This time shift can be expressed in terms of a phase angle, or “phaserelationship” between the quantities. There is an oscillation amplitudeassociated with each oscillating quantity, and together the angle andphase can be expressed as a vector. This vector can be decomposed intocomponents for the purpose of analysis.

Optionally, the method may further comprise determining the magnitude ofthe component of the second quantity that is in quadrature with thefirst quantity.

Components of a signal that are in quadrature are orthogonal withanother signal. That is they may lead or lag the other signal by 90degrees. The applicant has identified that the contribution of a gridsubsystem to oscillations in grid frequency in an external electricalgrid may be determined by determining the component of oscillations inactive power that lead or lag oscillations in grid frequency.

Optionally, the first quantity may comprise oscillations in gridfrequency in the grid subsystem.

Often the grid subsystem may be more accessible than elements of theexternal electrical grid. This may be because the certain elements ofthe external electrical grid may be located in other territories orcountries. Therefore advantages of ease of measurement arise when thefirst quantity corresponds to oscillations in grid frequency in the gridsubsystem.

Measurements of the oscillations in grid frequency in the grid subsystemmay correspond to the oscillations in grid frequency in the externalelectrical grid by denoting the type of grid oscillation as explainedbelow. That is, if the type of grid oscillation is common mode then theoscillations in grid frequency in the external electrical grid will bethe same phase as the oscillations in grid frequency in the gridsubsystem. If the type of grid oscillation is opposing phase then theoscillations in grid frequency in the external electrical grid will bein opposing phase to the oscillations in grid frequency in the gridsubsystem. In this way, measurements of grid frequency in both the gridsubsystem and the external electrical grid correspond to oscillations ingrid frequency in the external electrical grid.

Optionally, the method may further comprise receiving a third quantitycomprising oscillations in grid frequency within the external electricalgrid.

Receiving measurements corresponding to oscillations in grid frequencywithin the external electrical grid allows the more robust determinationof the contribution to the oscillations as explained below.

Optionally, the method may further comprise measuring the first quantitywithin the grid subsystem, measuring the second quantity on the at leastone electrical connection to the grid subsystem and measuring the thirdquantity within the external electrical grid.

Optionally, the method may further comprise determining whetheroscillations in grid frequency in the grid subsystem are in common modeto oscillations in grid frequency in the external electrical grid independence on the frequency of the first quantity being less than 0.1Hz.

Optionally, the method may further comprise determining whetheroscillations in grid frequency in the grid subsystem are in common modeto oscillations in grid frequency in the external electrical grid independence on the first quantity being the same amplitude and phase asthe third quantity.

Determining that the type of the oscillations of grid frequency arecommon mode allows the determination of whether a grid subsystem iscontributing positively or negatively to oscillations in grid frequencybased on the phase relationship of the first quantity to the secondquantity. In common mode if a component of the second quantity leads thephase of the first quantity then the grid subsystem is determined to benegatively contributing to the oscillations.

Optionally, the method may further comprise determining the magnitude ofa component of the second quantity leading the phase of the firstquantity by 90 degrees.

Optionally, the method may further comprise determining whetheroscillations in grid frequency in the grid subsystem are in opposingphase to oscillations in grid frequency in the external electrical gridin dependence on the frequency of the first quantity being greater than0.2 Hz.

Optionally, the method may further comprising determining whetheroscillations in grid frequency in the grid subsystem are in opposingphase to oscillations in grid frequency in the external electrical gridin dependence on the first quantity being 180 degrees out of phase tothe third quantity.

Determining that the type of oscillations of grid frequency is opposingphase allows the determination of whether a grid subsystem iscontributing positively or negatively to oscillations in grid frequencybased on the phase relationship of the first quantity to the secondquantity. In opposing phase if a component of the second quantity lagsthe phase of the first quantity when the first quantity comprisesoscillations in grid frequency in the grid subsystem then the gridsubsystem is determined to be negatively contributing to theoscillations.

Optionally, the method may further comprise determining a magnitude of acomponent of the second quantity lagging the phase of the first quantityby 90 degrees.

Optionally, the at least one electrical connection may comprises aplurality of electrical connections between the grid subsystem and theexternal electrical grid, and the second quantity may correspond to asummation of the oscillations in active power conveyed on each of theplurality of electrical connections.

Optionally, at least one electrical connection may comprise a pluralityof electrical connections between the grid subsystem and the externalelectrical grid and receiving at least one second quantity may comprisereceiving a plurality of second quantities each second quantitycorresponding to oscillations in active power conveyed on one of theplurality of electrical connections, and determining at least one phaserelationship may comprise determining a plurality of phase relationshipsbetween the first quantity and the plurality of second quantities todetermine the components of the plurality of second quantities thatcontribute to oscillations in grid frequency in the external electricalgrid

Optionally, the method may further comprise determining the magnitudesof the components of the second quantities that are in quadrature withthe first quantity.

Optionally, the method may further comprise summing the magnitudes ofthe components of the second quantities that are in quadrature with thefirst quantity.

According to the disclosure in a second aspect there is provided amethod of determining the contribution of a grid subsystem of aplurality of grid subsystems to oscillations in grid frequency in anexternal electrical grid in an electrical power network, the methodcomprising: determining the contribution of a first grid subsystem ofthe plurality of grid subsystems according to the above method;determining the contribution of a second grid subsystem according to theabove method, wherein the magnitude of the contribution of the firstgrid subsystem is greater than the magnitude of the contribution of thesecond grid subsystem; and normalising the magnitude of the second gridsubsystem to the magnitude of the first grid subsystem.

Optionally the method may further comprise determining that the secondgrid subsystem is contributing to the oscillations in grid frequency inthe external electrical grid if the normalised magnitude of the secondgrid subsystem is 0.5 or more.

According to the invention in a third aspect there is provided acomputer program product comprising computer program code executable ona computer processor to carry out the method described above.

According to the disclosure in a fourth aspect there is provided anapparatus for determining the contribution of a grid subsystem tooscillations in grid frequency in an external electrical grid in anelectrical power network, the apparatus comprising: a processorconfigured to receive a first quantity corresponding to the oscillationsin grid frequency in the external electrical grid, receive at least onesecond quantity, the at least one second quantity corresponding tooscillations in active power conveyed on at least one electricalconnection between the grid subsystem and the external electrical grid,and determine at least one phase relationship between the first quantityand the at least one second quantity to determine the component of thesecond quantity that contributes to oscillations in grid frequency inthe external electrical grid.

Optionally, the processor may be further configured to determine amagnitude of the component of the second quantity that is in quadraturewith the first quantity.

Optionally, the apparatus further comprises a first measurement unit incommunications link to the processor and to the at least one electricalconnection to the grid subsystem, the first measurement unit arranged tomeasure the first and/or second quantities and provide them to theprocessor.

Optionally, the first measurement unit may comprise a phasor measurementunit.

According to the disclosure in a fifth aspect there is provided anelectrical grid comprising the apparatus described above.

When referring to oscillations in grid frequency in the first and secondgrid areas being 180 degrees out of phase, it is noted that thisencompasses oscillations in grid frequency that are approximately 180degrees out of phase. For example, the oscillations in grid frequencymay be 180 degrees out of phase +/−45 degrees.

According to the disclosure in a fifth aspect there is provided anelectrical grid comprising: a grid subsystem; measurement apparatus thatis operable to measure a first quantity that corresponds to gridfrequency on an electrical connection to the grid subsystem over timeand to measure a second quantity that corresponds to active powerconveyed on an electrical connection from the grid subsystem over time,each of the first quantity and the second quantity changing in anoscillatory fashion over time; and processing apparatus that is operableto determine a phase relationship between the first and secondoscillating quantities.

In use, the measurement apparatus measures the first quantity, e.g. thegrid frequency, and the second quantity, e.g. the active power. Thefirst quantity, which corresponds to the oscillating grid frequency,oscillates at a frequency, e.g. below the nominal grid frequency at afrequency such as at 0.06 Hz, that is characteristic of gridoscillation. The second quantity oscillates in a fashion that is alsocharacteristic of the grid oscillation. Hence it is the change, i.e.oscillation, in each of the first and second quantities over time, thatis operated upon by the processing apparatus.

More specifically, the first quantity may be the grid frequency.Alternatively, the first quantity may be an angle measurement. Anoscillation in the angle of a measured voltage is equivalent to anoscillation in grid frequency measured at substantially the samelocation, e.g. at the same bus, with a phase shift of 90 degrees.Alternatively the first quantity may be the angle of a generator rotor,although the same 90 degree phase shift is required. Alternatively, thefirst quantity may be generator rotor speed. The generator rotor mayrotate at a frequency corresponding to the grid frequency and thereforeoscillations in grid frequency may be apparent in the speed of thegenerator rotor.

Alternatively or in addition, the second quantity may be active power.The active power may be derived from voltage measurements, e.g. at abus, and current measurements, e.g. in a circuit connecting to the bus.Alternatively or in addition, the second quantity may be current, e.g.as measured in a circuit connecting to the bus. Therefore, a measuredcurrent may correspond to oscillations in active power.

The processing apparatus is operative to determine the phaserelationship between the oscillating first and second quantities. Forexample, the processing apparatus may be operative to determine if thereis a phase difference between the oscillating first and secondquantities.

An oscillation in frequency in an electrical grid may represent anoscillation in the rotational speed of rotating machines, e.g.generators, within the grid. The oscillation involves periodicacceleration and deceleration of the grid, or some parts of it, androtating machines are accelerating and decelerating together with thefrequency oscillation. These cyclic changes in rotational speed occurbecause of cyclic torques acting on the rotating components. Thepresence of oscillations in the speed of rotating masses in a grid orsubsystem may mean that there are oscillations in mechanical and/orelectrical power supplied to, or exported from the subsystem. Dependingon the physical properties of the constituent parts, and thecontribution of control systems, a subsystem of the grid may supplyenergy to sustain the oscillation or remove energy from theoscillations.

By measuring the oscillations in the first quantity, e.g. the gridfrequency, at the subsystem and in the second quantity, e.g. the powerflow from the subsystem to the wider grid, it is possible to identifywhether the subsystem is improving or degrading the stability of theoscillations. In the case of common mode oscillations, if theoscillations in the second quantity lead the oscillations in the firstquantity by between zero and 180 degrees, the subsystem is degrading thestability of the oscillation by supplying energy to sustain theoscillation. Hence, the operating company can carry out investigationson the grid subsystem to determine the nature of the problem. If theoscillations in the second quantity lag the oscillations in the firstquantity by between zero and 180 degrees, the subsystem is improving thestability of the oscillation by extracting energy from the oscillationand providing damping of the oscillation. Hence, the operating companyneed not carry out investigations on the grid subsystem. Alternativelyand where there is a cross boundary issue involving, for example, two ormore operating companies, an operating company can demonstrate that itsnetwork area is not contributing to the grid oscillation. The extent ofthe positive or negative contribution may be related to the amplitude ofthe power swings and also how close the phase difference is to 90degrees.

Hence, alternatively or in addition, the processing apparatus may beoperable to determine if one of the first and second quantities leadsthe other of the first and second quantities.

Alternatively or in addition, the measurement apparatus may comprise aphasor measurement unit (PMU) that is operative to measure the first andsecond quantities on the electrical connection. Thus, the PMU may beoperative to measure the oscillating grid frequency and active powersignals.

The first quantity can be measured in a number of ways. Morespecifically, if the subsystem comprises a generation plant, the firstquantity can be measured by measuring the speed of at least one rotatingapparatus, such as a rotating shaft, turbine or generator of thegeneration plant. Where the generation plant comprises a plurality ofgenerating units, an average of speed measurements at each of therotating apparatus may be determined. Alternatively or in addition, thefirst quantity can be derived from measurement of a sinusoidal voltagewaveform, e.g. at a bus. Alternatively or in addition, a measured firstquantity may be synchronised with an external time reference, e.g. aGlobal Positioning System (GPS) time source.

Alternatively or in addition and where the subsystem comprises aplurality of generators, the first quantity may be measured at each of aplurality of different locations to provide plural first quantitymeasurements. More specifically, the oscillating first quantity operatedupon by the processing apparatus may be determined by one of: averagingthe plural first quantity measurements; determining a weighted averageof the plural first quantity measurements; and selection of a singlemost appropriate first quantity measurement from the plural firstquantity measurements.

Alternatively or in addition, the measurement apparatus may comprise ananalogue to digital converter that is operative such that measured firstand second quantities are output in a digital form. For example, themeasurement apparatus may be operative to measure voltage and currentwaveforms, to convert each of the measured voltage and current signalsto a digital form and to derive the first and second quantities from thedigital voltage and current signals.

Alternatively or in addition, the processing apparatus may be operativeto determine an extent of a phase difference between the measuredoscillating first and second quantities.

Alternatively or in addition, the processing apparatus may comprisecross correlation apparatus that is operative to correlate theoscillating first and second quantities with each other. Thus, the crosscorrelation apparatus may be operative to determine an extent of a phasedifference between the oscillating first and second quantities.

Alternatively or in addition, the processing apparatus may compriseoutput apparatus that is operable to provide an output to a user. Morespecifically, the output apparatus may be operable to provide a phaserelationship between the first and second oscillating quantities to theuser. The user may then draw one of the following conclusions from thephase relationship:

-   -   The subsystem is degrading the stability of the grid with        respect to the type of oscillation.    -   The subsystem is responding to an oscillation in the grid and is        contributing to the damping of the oscillation.    -   The subsystem is responding to an oscillation in the grid but is        providing no positive or negative contribution to the damping of        the oscillation.    -   The oscillation is transferred through a network of the        subsystem, but the subsystem is not substantially responding in        any way.

Alternatively or in addition, a frequency of oscillation of the firstquantity and of the second quantity may be less than a grid frequency,e.g. a grid frequency of 50 Hz or 60 Hz.

More specifically, the frequency of oscillation may be less thansubstantially 10 Hz. More specifically, the frequency of oscillation maybe less than substantially 5 Hz. More specifically, the frequency ofoscillation may be less than substantially 100 mHz. Alternatively or inaddition, the frequency of oscillation may be between substantially0.003 Hz and substantially 2 Hz.

Alternatively or in addition, the grid subsystem may comprise one of: apower plant, such as a generator; a control area; and an electricitynetwork area. The electricity network area may comprise at least onepower plant or control area. The term ‘control area’ as used herein maymean the area of an electric grid that a transmission system operatorhas the responsibility for controlling.

The subsystem may be interconnected to the wider system by any number ofelectric transmission lines. The second quantity, e.g. active power,used according to the disclosure may be representative of theoscillation in the sum total of, e.g. the active power from thesubsystem to the external electrical grid. The second quantity may beadequately represented by summing the second quantities in maintransmission paths. Hence, alternatively or in addition, the electricalgrid may comprise a plurality of electrical connections, e.g. maintransmission paths, to the grid subsystem. For example, a firstelectrical connection may connect the subsystem to another subsystem anda second electrical connection may connect the subsystem to yet anothersubsystem. More specifically, the electrical grid may comprise aplurality of measurement apparatus, each measurement apparatus beingoperable to measure at least one of the first and second quantities on adifferent one of the plurality of electrical connections. Morespecifically, the processing apparatus may be operative: to determine anextent of contribution between at least one of the first and secondquantities for each electrical connection; and to sum the determinedextents. Thus, where there is a plurality of electrical connections to agrid subsystem it may be possible to determine if the grid subsystem iscontributing positively or negatively to the stability of gridoscillation.

Alternatively or in addition, the electrical grid may be an electricpower network.

According to a sixth aspect of the present disclosure there is provideda method of determining whether or not a grid subsystem in an electricalgrid is contributing to grid oscillation, the method comprising:measuring a first quantity corresponding to grid frequency on anelectrical connection to the grid subsystem over time and a secondquantity corresponding to active power conveyed on an electricalconnection to the grid subsystem over time by way of measuringapparatus, each of the first and second quantities changing in anoscillatory fashion over time; and determining by way of processingapparatus a phase relationship between the first and second oscillatingquantities.

Embodiments of the sixth aspect of the present disclosure may compriseone or more features of the fifth aspect of the present disclosure.

According to a seventh aspect of the present disclosure there isprovided a kit of parts configured to be installed in an electrical gridcomprising a grid subsystem, the kit of parts comprising: measurementapparatus that upon installation is configured to and operable tomeasure a first quantity corresponding to grid frequency on anelectrical connection to the grid subsystem over time and a secondquantity corresponding to active power conveyed on an electricalconnection electrical connection to the grid subsystem over time, eachof the first and second quantities changing in an oscillatory fashionover time; and processing apparatus that upon installation is operableto determine a phase relationship between the first and secondoscillating quantities.

Embodiments of the seventh aspect of the present disclosure may compriseone or more features of the fifth aspect of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

Further features and advantages of the present disclosure will becomeapparent from the following specific description, which is given by wayof example only and with reference to the accompanying drawings, inwhich:

FIG. 1 is a graph of grid frequency over time that is subject to theeffects of grid oscillation;

FIG. 2 is a schematic diagram of a grid subsystem connected to anelectrical grid;

FIG. 3 shows a method of determining the contribution of a gridsubsystem to oscillations in grid frequency in an external electricgrid;

FIG. 4A is a graph in which the oscillating active power lags theoscillating grid frequency;

FIG. 4B is a graph in which the oscillating active power leads theoscillating grid frequency; and

FIG. 5a-c shows illustrative displays that may be provided by a deviceaccording to the disclosure.

SPECIFIC DESCRIPTION

It is noted that the unstable oscillations of FIG. 1 should bedistinguished over stable oscillations, which are oscillations that aredamped and remain small. Stable oscillations may be damped inherently bythe elements of the electrical grid without input from an operator. Anoperator can indirectly influence the damping of the electrical grid bychanging the generation dispatch and therefore the power flows in thenetwork. With appropriate information, an operator can improve thestability of the system by changing generation outputs.

Unstable oscillations grow in amplitude to a level that threatens thesecurity of an electrical power network whereas stable oscillationsremain small and do not necessarily threaten the security of anelectrical power network.

The method of the invention applies in both cases and may be used todetermine the contributions of grid subsystems to unstable oscillationsin grid frequency and/or stable oscillations in grid frequency. FIG. 1shows an example of unstable oscillations in grid frequency.

Generally, oscillations in grid frequency may be split into two groups:common mode frequency oscillations; and opposing phase frequencyoscillations.

Common mode frequency oscillations occur when the entire grid frequencyoscillates with substantially a coherent phase, i.e. in phase. If thefrequency is measured anywhere in the grid practically the sameoscillation would be seen. Both the amplitude and the phase of theoscillations are substantially the same throughout the grid. Common modefrequency oscillations where the frequency oscillation is in phasethroughout the system tend to be at very low frequency, e.g. 0.06 Hz.

The skilled person will appreciate that two measured oscillatingquantities are rarely precisely “in phase”. For example, errors inherentin measuring techniques may mean that two signals that are in phase willnot be measured as being exactly in phase. In addition, it is rare in anelectrical power network that signals that are “in phase” exhibitexactly the same phase. Indeed, the skilled person will appreciate that“in phase” is a term that may be applied to two signals that aresubstantially in phase as explained below.

Opposing phase frequency oscillations occur where there exist two groupsof generators operating in two areas of an electrical power network.Within a group generator frequencies oscillate roughly in phase witheach other, but from one group to another there is a phase shift infrequency oscillations of about 180 degrees. Opposing phase frequencyoscillations generally occur at a slightly higher frequency than commonmode frequency oscillations, typically between 0.1 Hz and 2 Hz.Inter-area electro-mechanical oscillations are an example of opposingphase oscillations in grid frequency occurring in different areas of anelectrical power network. Similarly, local mode electro-mechanicaloscillations are also opposing phase oscillations, but the amplitude ofthe oscillations in opposite phase are much smaller than the swings inthe main participating generator or generators.

Thus, with opposing phase frequency oscillations, if an oscillation infrequency is observed in the grid area of the first group of generators,it would be possible to find a location in the grid area of the opposinggroup where the frequency oscillation was substantially in opposingphase. There may be a different amplitude, but the shape of onefrequency oscillation waveform is roughly a scaled mirror image of theother.

As with the common mode definition provided above, two signals inopposing phase are rarely precisely 180 degrees out of phase. Indeed,the skilled person will appreciate that characteristics of an electricalpower network may determine that opposing phase is a term that may beapplied to two signals that are substantially 180 degrees out of phase.

Inter-area oscillations are an example of opposing phase oscillations inwhich the opposing phase of the oscillations in grid frequency occurringin different areas of an electrical power network. An example ofinter-area phase frequency oscillations is that the generators inScotland may be in a first grid area and oscillate together as a group,and the generators in the north of England may be in a second grid areaand also oscillate together to form a group. The two groups may swing inopposite phase at say approximately 0.5 Hz. In other examples the gridareas can be very large. There are modes of frequency oscillation thattraverse the whole of Europe, with Spanish, Portuguese and Frenchgenerators oscillating in opposite phase from the Balkans and EasternEurope.

It is noted that when referring to phase lead or lag of poweroscillations, the convention used herein is of positive power flow beingfrom a grid subsystem to a grid.

Referring to FIG. 2 there is shown a schematic representation of anelectrical power network 10. FIG. 2 shows an electrical power network 10in which it may be determined whether a grid subsystem 12 iscontributing to oscillations in grid frequency.

The electrical power network 10 comprises a grid subsystem 12 and anexternal electrical grid 14. The grid subsystem 12 may be one or morepower generators. The external electrical grid 14 may comprise furtherpower generators and/or control areas interconnected within the externalelectrical grid.

The term “external electrical grid” 14 relates to all elements of theelectrical power network that are separate or external to the gridsubsystem 12.

The grid subsystem 12 is connected to the external electrical grid 14 bymeans of electrical connections (or transmission lines) 16 a, 16 b, 16c. Frequency measurement units 18 a, 18 b, 18 c, which may be phasormeasurement units (PMUs), are operable to measure a frequency of avoltage present in the grid subsystem 12. The PMUs 18 a, 18 b, 18 c maybe positioned at the end of the transmission lines 16 a, 16 b, 16 crespectively and may therefore be positioned just within or at theboundary of the grid subsystem 12. However, the skilled person willunderstand that the PMUs 18 a, 18 b, 18 c may be placed in otherlocations within the grid subsystem 12 suitable for measuring thefrequency of the voltage within the grid subsystem 12.

The frequency of the voltage waveform oscillates or varies over time andthe measured variation may constitute a first quantity.

The frequency and active power measurements from the PMUs areelectrically conveyed to a central location 32. This is shownschematically in FIG. 2 by the communications connection 20 from PMU 18a to the central location 32. The communications connection 20 may be awired or wireless communication connection, an optical data connectionor any other form of data communications connection as will be known tothe skilled person. The central location 32 contains a computer having acentral processor 36, data storage 38 and an output device 40. Thecentral processor 36 is configured to perform digital signal processingoperations, such as cross-correlation. The design and implementation ofthe aforementioned electronic apparatus present in the central locationand firmware required to perform the data processing, includingcross-correlation, is readily within the scope of the ordinary designskills of the skilled person.

The frequency and power measurements taken by the PMUs 18 a, 18 b, 18 care conveyed to the processor 36 via communications connection 20.

In addition, one or more measurement units may be located within theexternal electrical grid 14. The measurement units within the externalelectrical grid 14 convey frequency measurements from within theexternal electrical grid 14 to the processor 36 via communicationsconnection 22.

Fundamental characteristics of an oscillation in grid frequency may beobserved in voltage phasor angle measurements or e.g. in generator rotorspeed oscillations, which can be used as a substitute for frequencymeasurements. In the case of voltage angle, there is generally a 90degree phase shift, but the concept still holds.

The PMUs 18 a, 18 b, 18 c may also be operable to measure variations inactive power, which may constitute a second quantity, conveyed on theelectrical connections 16 a, 16 b, 16 c.

The quantities measured with the PMUs 18 a, 18 b, 18 c are synchronised.Synchronisation may be achieved within a PMU or may be achieved with anexternal time reference, such as from a GPS time source. Each of thePMUs 18 a, 18 b, 18 c may be an AREVA P847 from AREVA T&D of St.Leonards Avenue, ST17 4 LX, Stafford, United Kingdom. Each of the PMUsis operative to output digital data.

The electrical power network 10 of FIG. 2 may be subject to anoscillation in grid frequency, e.g. a common mode oscillation of theform represented in FIG. 1 or an opposing phase oscillation.

The grid frequency in the grid subsystem is measured by the PMU 18 a.Further, the grid frequency in the external electrical grid 14 ismeasured at a location within the external electrical grid 14. Thelocation at which the grid frequency in the external electrical grid 14is measured is not shown in FIG. 2. In general, the location of such ameasurement should be far away from the grid subsystem 12 so that it isless likely to be influenced by the oscillations in the grid subsystem12.

The measurement unit(s) located in the external electrical grid 14 maybe PMUs. However, other measurement units that are within the knowledgeof the skilled person and are suitable for measurement of grid frequencymay be used. Additionally, the skilled person will appreciate that theremay be a plurality of frequency measurements recorded in the gridsubsystem 12 and/or the external electrical grid 14.

The PMU 18 a may be configured to measure the frequency of the voltagein the grid subsystem 12. The frequency of the voltage in the gridsubsystem may then be transmitted to the processor 36. From thefrequency of the voltage waveform the processor 36 is able to measurethe oscillation in grid frequency within the grid subsystem over time.It will be apparent to the skilled person that other methods ofmeasuring oscillation in grid frequency may be used within the scope ofthe present disclosure.

If there is more than one available measurement of grid frequency in thegrid subsystem 12 the most important may be selected for use. The mostimportant measurement may for example be the measurement exhibiting thelargest oscillation in frequency within the subsystem 12.

Similarly, the measurement unit located in the external electrical grid14 may be configured to measure the frequency of the voltage in theexternal electrical grid 14. The frequency of the voltage is conveyed tothe processor 36 via a communications connection 22. The communicationsconnection 22 may be a wired or wireless communication connection, anoptical data connection or any other form of data communicationsconnection as will be known to the skilled person. From the frequency ofthe voltage the processor 36 is able to calculate the oscillation ingrid frequency within the external electrical grid over time. It will beapparent to the skilled person that other methods of measuringoscillation in grid frequency may be used within the scope of thepresent disclosure.

Using the extracted oscillation in grid frequency in the grid subsystem12 and the extracted oscillation in grid frequency in the externalelectrical grid 14, the processor may be configured to determine whetherthe oscillations in the grid frequency in the grid subsystem 12 and theexternal electrical grid 14 are common mode or opposing phase.

This may be done by comparing the amplitude and phase of themeasurements of oscillations in grid frequency in the grid subsystem 12to the amplitude and phase of the measurements of oscillations in gridfrequency in the external electrical grid 14. If both sets ofmeasurements are substantially similar in amplitude and phase, then itmay be determined that the oscillations in grid frequency are commonmode. Otherwise, the oscillations in grid frequency are considered to beopposing phase.

In the common mode case, when comparing the oscillations in gridfrequency in the grid subsystem 12 and the oscillations in gridfrequency in the electrical grid 14 errors inherent in the measurementof the two quantities may mean that the frequency oscillations are notprecisely the same phase. In addition, the characteristics of theelectrical power network 10 may be such that signals that are notexactly in phase may still be considered to be in common modeoscillation.

There must be a phase shift between the frequencies in differentlocations, otherwise there would be no oscillating power flow and theoscillation would not be sustained. This phase difference tends to besmall, e.g. less than 10 degrees.

For example, in some embodiments the phase of the oscillations in gridfrequency in the grid subsystem 12 and the external electrical grid 14may be in common mode oscillation if they are the same phase plus orminus 45 degrees. Alternatively, the phase of the oscillations in gridfrequency in the grid subsystem 12 and the external electrical grid 14may be in common mode oscillation if they are the same phase plus orminus 30 degrees. Alternatively, the phase of the oscillations in gridfrequency in the grid subsystem 12 and the external electrical grid 14may be in common mode oscillation if they are the same phase plus orminus 20 degrees. Alternatively, the phase of the oscillations in gridfrequency in the grid subsystem 12 and the external electrical grid 14may be in common mode oscillation if they are the same phase plus orminus 10 degrees. Alternatively, the phase of the oscillations in gridfrequency in the grid subsystem 12 and the external electrical grid 14may be in common mode oscillation if they are the same phase plus orminus 5 degrees.

Additionally, similar phenomena may be seen when comparing measurementsof the amplitude of the oscillations in grid frequency. Therefore, theoscillations in grid frequency in the grid subsystem 12 and the externalelectrical grid 14 may be considered the same for determining commonmode oscillation if they are the same amplitude plus or minus 20%.Alternatively, the amplitude of the oscillations in grid frequency inthe grid subsystem 12 and the external electrical grid 14 may beconsidered the same for determining common mode oscillation if they arethe same amplitude plus or minus 10%. The amplitude of the oscillationsin grid frequency in the grid subsystem 12 and the external electricalgrid 14 may be considered the same for determining common modeoscillation if they are the same amplitude plus or minus 5%.

In the opposing phase case, when comparing the oscillations in gridfrequency in the grid subsystem 12 and the oscillations in gridfrequency in the electrical grid 14 errors inherent in the measurementof the two quantities may mean that the frequencies are not precisely180 degrees out of phase. In addition, the characteristics of theelectrical power network 10 may be such that signals that are not 180degrees out of phase may still be considered to be in opposing phaseoscillation. Considering that the angles relate to the relativepositions of generator rotors of different sizes and types of machinesconnected through electrical transmission lines, an exactly symmetricalopposing phase oscillation may be very unlikely in an electric powernetwork.

For example, in some embodiments the phase of the oscillations in gridfrequency in the grid subsystem 12 and the external electrical grid 14may be in opposing phase oscillation if they are 180 degrees out ofphase plus or minus 45 degrees. Alternatively, the phase of theoscillations in grid frequency in the grid subsystem 12 and the externalelectrical grid 14 may be in opposing phase oscillation if they are 180degrees out of phase plus or minus 30 degrees. Alternatively, the phaseof the oscillations in grid frequency in the grid subsystem 12 and theexternal electrical grid 14 may be in opposing phase oscillation if theyare 180 degrees out of phase plus or minus 10 degrees. Alternatively,the phase of the oscillations in grid frequency in the grid subsystem 12and the external electrical grid 14 may be in opposing phase oscillationif they are 180 degrees out of phase plus or minus 5 degrees. The term“opposing phase” therefore applies to frequency measurementscorresponding to any of the above examples.

It is also possible to determine whether the oscillations in gridfrequency are common mode or opposing phase without any comparison ofoscillations in grid frequency measurements from the grid subsystem 12and the external electrical grid 14. For example, if calculatedoscillations in grid frequency are only available from the gridsubsystem or the external electrical grid then the type of oscillationmay be inferred.

In such circumstances the type of the oscillations in grid frequency maybe determined on the basis of the frequency of the calculatedoscillations in grid frequency.

That is, if the calculated oscillation in grid frequency within the gridsubsystem 12 or the external electrical grid 14 is less than 0.2 Hz thenthe oscillations in grid frequency can be determined to be common modethroughout the entire electrical power network 10 including thesubsystem 12. Alternatively, if the calculated oscillation in gridfrequency within the grid subsystem 12 or the external electrical grid14 is less than 0.1 Hz then the oscillations in grid frequency can bedetermined to be common mode throughout the entire electrical powernetwork 10 including the subsystem 12. Alternatively, if the calculatedoscillation in grid frequency within the grid subsystem 12 or theexternal electrical grid 14 is less than 0.06 Hz then the oscillationsin grid frequency can be determined to be common mode throughout theentire electrical power network 10 including the subsystem 12.

Otherwise, if the oscillations in grid frequency measured within thegrid subsystem 12 or the external electrical grid 14 are greater than0.2 Hz then the oscillations in grid frequency can be determined to beopposing phase. In other embodiments, if the oscillations in gridfrequency measured within the grid subsystem 12 or the externalelectrical grid 14 are greater than 0.1 Hz then the oscillations in gridfrequency can be determined to be opposing phase.

Therefore it is possible to infer from measurements in a grid subsystemthat there is a part of the grid oscillating at approximately 180degrees out of phase to the grid subsystem, even if no measurements aretaken showing this. In practice, if an oscillation is observed in a gridsubsystem with a significant/measurable amplitude at one location, andthe oscillation is not measurable, or is measurable with a much smalleramplitude at another location, then it can be inferred that somewhere inthe grid there is an opposing frequency oscillation.

The threshold for determining whether oscillations are common mode oropposing phase may be set between 0.1 Hz and 0.2 Hz. Oscillations belowthe threshold are considered to be common mode and oscillations abovethe threshold are considered to be opposing phase.

The same principles may also be applied if a measurement in the externalelectrical grid 14 is close to the grid subsystem 12.

Active power may be measured on the electrical connection 16 a betweenthe grid subsystem 12 and the external electrical grid 14. The activepower measurement may be taken by the PMU 18 a and conveyed to thecentral processor 36. The central processor may be configured tocalculate (or extract) the oscillations in active power over time.

The central processor 36 is operative to cross-correlate the calculatedoscillations in grid frequency and oscillations in active power signalsfrom the PMU 18 a to determine a phase relationship between theoscillations in active power and the oscillations in grid frequency. Inone embodiment, the central processor 36 may be configured to determinea magnitude of the component of oscillations in power that is inquadrature with the oscillations in grid frequency.

Also, the central processor 36 may be operative to cross-correlate thecalculated oscillations in grid frequency and oscillations in activepower signals from other PMUs to determine a magnitude of the componentof oscillations in power that is in quadrature with the oscillations ingrid frequency.

If it is determined that the oscillations in grid frequency are commonmode then the contribution of the grid subsystem 12 to the oscillationsin grid frequency can be determined by determining the magnitude of theoscillations in active power that are in quadrature with theoscillations in grid frequency. That is, the magnitude of the componentof the oscillation in active power that leads the oscillations in gridfrequency by 90 degrees. The central processor 36 is thereforeconfigured to determine the contribution of the grid subsystem to theoscillations in grid frequency in this way if it is determined that theoscillations in grid frequency are in common mode.

The contribution of a grid subsystem 12 to the oscillations in gridfrequency in the external electrical grid 14 may be measured in power.

The contribution may be determined as the component of a vectorrepresenting oscillations in active power in quadrature, i.e. at 90degrees, to a vector representing oscillations in grid frequency in thegrid subsystem or the external electrical grid.

The phase angle between the vector representing oscillations in activepower and the vector representing oscillations in grid frequency may beused together with the magnitude of the vector representing oscillationsin active power in order to derive the contribution. The contributionmay be determined asContribution=P sin(delta)

Where P is the magnitude of the vector representing oscillations inactive power and delta is the phase angle between the vectorrepresenting oscillations in active power and the vector representingoscillations in grid frequency.

This is the component of power oscillation in quadrature to thefrequency oscillation.

The phase angle may also be used without magnitude to identify whetherthe subsystem is contributing positively or negatively to theoscillation.

For the avoidance of doubt it is noted that if the oscillations inactive power lead the oscillations in grid frequency by between 0 and180 degrees a component of the oscillations in active power will leadthe oscillations in grid frequency by 90 degrees.

If it is determined that the oscillations in grid frequency are inopposing phase then the contribution of the grid subsystem 12 to theoscillations in grid frequency can be determined by determining themagnitude of the oscillations in active power that are in quadraturewith the oscillations in grid frequency in the external electrical grid14. That is the magnitude of the component of the oscillation in activepower that leads the oscillations in grid frequency in the externalelectrical grid 14 by 90 degrees. The central processor 36 is thereforeconfigured to determine the contribution of the grid subsystem to theoscillations in grid frequency in this way if it is determined that theoscillations in grid frequency are in opposing phase.

The skilled person will appreciate that if the grid oscillations are inopposing phase then the oscillations of the grid subsystem 12 aresubstantially 180 degrees out of phase with the oscillations of theexternal electrical grid 14. Therefore the magnitude of the component ofthe oscillations in active power that leads the oscillations in gridfrequency in the external electrical grid 14 by 90 degrees correspondsto the magnitude of the component of the oscillations in active powerthat lags the oscillations in grid frequency in the grid subsystem 12 by90 degrees.

Further, either the oscillations in grid frequency in the grid subsystem12 or the oscillations in grid frequency in the external electrical grid14 may be inferred as described above. Therefore, the contribution ofthe grid subsystem 12 to the oscillations in grid frequency in theexternal electrical grid may be determined by comparison of theoscillations in active power with an inferred estimate of theoscillations in grid frequency.

The result, i.e. positive contribution, negative contribution or nocontribution, and the extent or magnitude of the contribution is thenoutput on the output device 40 whereupon the operator may takeappropriate action. The nature of the phase relationship betweenoscillations in grid frequency and oscillations in active power isdescribed below with reference to FIGS. 4A and 4B, which specify agenerator as the grid subsystem.

FIG. 3 shows a method of the contribution of a grid subsystem 12 tooscillations in grid frequency in an external electrical grid 14 in anelectrical power network 10. The method comprises receiving a firstquantity 42 at a processor 36.

The first quantity corresponds to oscillations in grid frequency in thegrid subsystem 12. That is the first quantity may be used to determinethe oscillations in the grid frequency in the grid subsystem. However,it is noted that the first quantity may correspond to oscillations ingrid frequency in the external electrical grid. That is the firstquantity may be used to determine the oscillations in the grid frequencyin the external electrical grid.

The first quantity may be the grid frequency of a voltage waveform inthe grid subsystem 12 recorded over time. Alternatively the firstquantity may be an angle measurement, i.e. a voltage phase anglemeasurement. Variations in the first quantity over time are thereforerepresentative of the oscillation in grid frequency.

The method also comprises receiving a second quantity 44 at theprocessor 36.

The second quantity corresponds to oscillations in active power conveyedon at least one of the electrical connections 16 a, 16 b, 16 c betweenthe external electrical grid 14 and the grid subsystem 12. That is, thesecond quantity may be used to determine the variation in the activepower over time. The second quantity may be active power derived frommeasurements of a voltage and a current recorded over time. The skilledperson will be aware of how to calculate active power based on thecombination of these measurements. The second quantity may alternativelybe a current recorded over time. The skilled person will be aware of howto calculate active power based on either of these measurements.Oscillations in the second quantity over time therefore representvariations in the active power, or alternatively, variations in thecurrent.

The method also comprises determining a phase relationship 46 betweenthe first and second quantities to determine the component of the secondquantity that contributes to oscillations in grid frequency in theexternal electrical grid. This phase relationship may be determined bythe processor 36. Determining a phase relationship may comprisedetermining whether there is a phase difference between the first andsecond quantities. Further, the processor 36 may be configured todetermine whether the oscillations in active power lead the oscillationsin grid frequency by between 0 and 180 degrees. Further, the processor36 may be configured to determine the magnitude of a component of theoscillations in active power that lead the oscillations in gridfrequency by 90 degrees.

If the oscillations in active power lead the oscillations in gridfrequency in the external electrical grid 14 by between 0 and 180degrees then the grid subsystem is contributing negatively to theoscillations in grid frequency in the external electrical grid 14. Thatis, the grid subsystem is supplying energy to sustain the oscillationsin grid frequency in the external electrical grid.

If the oscillations in active power lag the oscillations in gridfrequency in the external electrical grid 14 by between 0 and 180degrees then the grid subsystem is contributing positively to theoscillations in grid frequency in the external electrical grid 14. Thatis, the grid subsystem is extracting energy in order to damp theoscillations in grid frequency in the external electrical grid.

The extent of the positive or negative effect is related to a magnitudeof a component of the oscillations in active power that lead or lag theoscillations in grid frequency of the external electrical grid bysubstantially 90 degrees.

That is, in the common-mode case (i.e. frequency oscillations in-phasethroughout a grid subsystem) a phase lead of the oscillations in activepower over the oscillations in grid frequency of the external electricalgrid 14 implies that the grid subsystem 12 is contributing tooscillations in grid frequency in the external electrical grid 14negatively (i.e. making them worse).

The method may further comprise measuring the first and/or secondquantities. This may be done using the measurement units 18 a, 18 b, 18c, which provide the measured quantities to the processor 36. Theoscillation in grid frequency and oscillation in active power may becalculated by analysing discrete time sequential measurements of thefirst quantity and second quantities respectively. The analysis may beperformed by a measurement unit 18 a, 18 b, 18 c or by the processor 36.

In some embodiments of the present disclosure, active power may bemeasured on a plurality of electrical connections 16 a, 16 b, 16 cbetween the grid subsystem 12 and the external electrical grid 14. Theplurality of measurements of active power may be taken by PMUs 18 a, 18b and 18 c respectively and conveyed to the central processor viacommunications connections (not shown).

If a plurality of measurements of active power is taken from theelectrical connections 16 a, 16 b, 16 c then the central processor 36may be configured to calculate (or extract) a plurality of oscillationsin active power, one relating to each electrical connection 16 a, 16 b,16 c. The second quantity may comprise a summation of all the calculatedoscillations in active power. In this case, the convention of power flowfrom the grid subsystem 12 to the external electrical grid 14 beingpositive is used. The central processor 36 may also be furtherconfigured to determine the contribution of the grid subsystem 12 in themanner described above using the summed active powers for the secondquantity.

In alternative embodiments a plurality of second quantities may comprisethe plurality of calculated oscillations in active power. The centralprocessor 36 may therefore be configured to determine a plurality ofphase relationships. In some embodiments, the central processor may beconfigured to determine a plurality of magnitudes of components ofoscillations in active power that are in quadrature to oscillations ingrid frequency. The central processor may be further configured todetermine a contribution of the grid subsystem to oscillations in gridfrequency by summing the plurality of magnitudes of components ofoscillations in active power that are in quadrature to oscillations ingrid frequency.

The above method provides an indication as to whether the first gridsubsystem 12 is contributing positively or negatively to the stabilityof oscillations in grid frequency in the electrical grid and the extentof that contribution.

In other embodiments of the present disclosure the electrical powernetwork may comprise a plurality of grid subsystems. In such embodimentsthe contribution of each grid subsystem to the oscillations in gridfrequency in the external electrical grid may be determined according tothe method set out above.

In such cases the magnitude of the contribution of each grid subsystemwill be different. The magnitude of the contribution of one gridsubsystem will be greater than the magnitude of the contribution of theother grid subsystems.

The applicants have realised that it is advantageous when a plurality ofgrid subsystems are contributing to oscillations in grid frequency in anexternal electrical grid to determine the contribution of each gridssubsystem relative to the other grid subsystems. Therefore, in the casewhere a first grid subsystem has the largest magnitude of contributionto the grid frequency oscillations then a magnitude of the contributionof a second (and subsequent) grid subsystem is normalised to thecontribution of the first grid subsystem.

That is, the magnitude of the contribution of the first grid subsystemis normalised to 1 and the magnitude of the contributions of the othergrid subsystems is amended accordingly to be a value between −1 and 1.For example if the magnitude of the contribution of the second gridsubsystem is half the magnitude of the contribution of the first gridsubsystem then the normalised values of the magnitude of thecontributions of the first and second grid subsystems would be 0.5 and 1respectively. As an additional example, if the magnitude of thecontribution of the second grid subsystem is half the magnitude (andnegative, i.e. in the opposite direction) of the contribution of thefirst grid subsystem then the normalised values of the magnitude of thecontributions of the first and second grid subsystems would be −0.5 and1 respectively.

In some embodiments of the present disclosure the second (andsubsequent) grid subsystems are considered to be contributing to theoscillations in grid frequency in the external electrical grid if thenormalise magnitude of the contribution is 0.5 or greater. That is, thecontribution of a grid subsystem to oscillations in grid frequency in anexternal electrical grid is only considered to be significant if thenormalised magnitude of the contribution of the grid subsystem is 0.5 orgreater.

FIG. 4A shows plots 50 of oscillations in grid frequency 52 by way ofthe dotted line and oscillations in active power 54 by way of the solidline against time. The oscillations in grid frequency in plot 52 areshown as deviation from the grid frequency, which in the present case is60 Hz. That is, 0 Hz in FIG. 4A represents a 60 Hz grid frequency. Theoscillations in active power in plot 54 are likewise detrended to showactive power oscillations around to 0 MW. Hence, the plots in FIG. 4Aonly show the oscillation in grid frequency and active power over time,and not the steady-state components that are caused by the oscillationin grid frequency. As can be seen, the active power plot lags the gridfrequency plot; this is indicative of the generator responding tooscillation in grid frequency.

FIG. 4B shows plots 60 of oscillations in grid frequency 62 by way ofthe dotted line and oscillations in active power 64 by way of the solidline against time. As with FIG. 4A, the plots in FIG. 4B are detrendedsuch that they only show the oscillations in grid frequency and activepower over time, and not the steady-state components that are caused bythe grid oscillation. As can be seen, the oscillations in the activepower plot lead the frequency plot; this is indicative of the generatorcontributing to oscillation in grid frequency.

The skilled person will appreciate that the measurement units (PMUs) 18a, 18 b, 18 c are not essential. The magnitude of contribution of a gridsubsystem may be determined by determining the phase relationshipbetween the first quantity and the second quantity measured by a PMU.Measurements of the first and second quantities may be provided to theprocessor 36 by other parties or means. For example, measurements offrequency and active power may be taken in other territories orcountries by third parties and conveyed to the central location 32 wherethe method of the present disclosure may be carried out.

Additionally, in embodiments where a measurement unit is used to measurefirst and second quantities on an electrical connection to a gridsubsystem, a single measurement unit may provide sufficient informationto determine whether a grid subsystem is contributing to oscillations ingrid frequency in an external electrical grid. In such embodiments,there is no requirement to sum quantities as mentioned above. The phaserelationship between the first and second quantity from the measurementunit is sufficient to determine whether there is a contribution. Themagnitude of the contribution may be determined from the magnitude of acomponent of the oscillations in active power that leads theoscillations in grid frequency by 90 degrees.

Referring to FIG. 5 a-c there is shown a series of display screens thatmay be output by the device 40. The display screens show maps ofdifferent regions with grid subsystems 92, 94, 96, 98 and variouselectrical connections to the grid subsystems. Arrows are shown on theelectrical connections. The size of each arrow corresponds to themagnitude of a contribution of the grid subsystem 12 to oscillations ingrid frequency in the external electrical grid 14.

Referring to FIG. 5a , the unshaded hexagon 90 indicates a frequencyoscillation measurement in a grid subsystem. This oscillation infrequency is defined as the phase reference, i.e. 0 degrees. The shadedhexagons 92 and 94 represent frequency oscillation measurements inopposing phase to the reference 90 in an external electrical grid.

Therefore, referring to FIG. 2, the unshaded hexagon 90 may be withinthe grid subsystem 12 and the shaded hexagons 92, 94 may be within theexternal electrical grid 14.

Electrical connection 100 has arrows 102, 104 displayed upon it. Thearrow 102 represents the influence of variations in active power on theoscillations in grid frequency of the external electrical grid. Thearrow 104 represents the influence of oscillations in active power inthe electrical connection 100 on the oscillations in grid frequency inthe grid subsystem. The size of arrows 102, 104 is determined by firstlydetermining a phase relationship between the oscillation in gridfrequency of the external electrical grid or the grid subsystem and theoscillation in active power conveyed on the electrical connection 100.The size of the arrows 102, 104 may correspond to the magnitude of thecomponent of the oscillations in active power that lead the oscillationsin grid frequency in the shaded and unshaded groups respectively, bysubstantially 90 degrees.

Referring to FIG. 5b , the electrical connections 106, 108 both connectto a grid subsystem 12. The grid subsystem 12 has a measurement ofoscillations in grid frequency recorded within the subsystem 96. Arrows110, 112 are displayed on connections 106, and arrows 114, 116 aredisplayed on connection 108. The size of the arrows is determined asdescribed above and represents the magnitude of the contribution of thegrid subsystem 12 to oscillations in the external electrical grid.

Therefore, a summation of arrows 112 (negative) and 116 (positive) showsthe contribution of grid subsystem 12 on the external electrical grid.Specifically, the magnitude of the contribution on connection 106corresponding to arrow 112 may be subtracted from the magnitude of thecontribution on connection 108 corresponding to arrow 116 to determinethe contribution of grid subsystem 12 on the oscillations in gridfrequency of the external electrical grid.

In the example shown in FIG. 5b the grid subsystem 12 is damping theoscillations in grid frequency in the external electrical grid assummation of the contributions illustrated by the arrows 112 and 116gives a negative result.

Referring to FIG. 5c , grid subsystem 12 does not contribute to theoscillations in grid frequency of the external electrical grid as shownby the relative sizes of arrows 112 and 124, which illustrate thecontributions of the grid subsystem, are substantially equal.

The present disclosure allows the determination and display of: the gridsubsystems, regions or plants contributing to an oscillation in gridfrequency in an external electrical grid; and the transmission corridorsor connections where power oscillations are influencing the oscillationsin grid frequency in the external electrical grid. The operator of anelectrical power network can then identify the grid subsystem, e.g.generator or group of generators, contributing to oscillations in gridfrequency and re-dispatch generation in the network such that the poweris reduced on the relevant connection.

The skilled person will be able to identify other embodiments of theinvention without departing from the scope of the appended claims.

The invention claimed is:
 1. A method of determining contribution of agrid subsystem to oscillations in grid frequency in an externalelectrical grid in an electrical power network, the method comprising:measuring, via at least one phasor measurement unit, oscillations ingrid frequency in the external electrical grid; measuring oscillationsin power conveyed on at least one electrical connection between the gridsubsystem and the external electrical grid; and determining thecontribution of the grid subsystem to the oscillations in the gridfrequency in the external electrical grid by determining at least onephase relationship between the oscillations in grid frequency and theoscillations in power conveyed to determine a component of theoscillations in power that contributes to the oscillations in gridfrequency in the external electrical grid.
 2. The method according toclaim 1 wherein determining the component of the oscillations in powerthat contribute to the oscillations in grid frequency in the externalelectrical grid comprises determining a magnitude of the component ofthe oscillations in power that is in quadrature with the oscillations ingrid frequency.
 3. The method according to claim 1 wherein theoscillations in grid frequency comprises oscillations in grid frequencyin the grid subsystem.
 4. The method according to claim 3 furthercomprising receiving a third quantity comprising oscillations in gridfrequency within the external electrical grid.
 5. The method accordingto claim 4 further comprising measuring the oscillations in gridfrequency within the grid subsystem, measuring the oscillations in powerconveyed on the at least one electrical connection to the grid subsystemand measuring the third quantity within the external electrical grid. 6.The method according to claim 5, further comprising determining whetheroscillations in grid frequency in the grid subsystem are in common modeto oscillations in grid frequency in the external electrical grid independence on the frequency of the oscillations in grid frequency beingless than 0.1 Hz.
 7. The method according to claim 6 further comprisingdetermining a magnitude of the component of the oscillations in powerleading the phase of the oscillations in grid frequency by substantially90 degrees.
 8. The method according to claim 4, further comprisingdetermining whether oscillations in grid frequency in the grid subsystemare in common mode to oscillations in grid frequency in the externalelectrical grid in dependence on the oscillations in grid frequencybeing substantially the same amplitude and phase as the third quantity.9. The method according to claim 4, further comprising determiningwhether oscillations in grid frequency in the grid subsystem are inopposing phase to oscillations in grid frequency in the externalelectrical grid in dependence on the oscillations in grid frequencybeing substantially 180 degrees out of phase to the third quantity. 10.The method according to claim 3, further comprising determining whetheroscillations in grid frequency in the grid subsystem are in opposingphase to oscillations in grid frequency in the external electrical gridin dependence on the frequency of the oscillations in grid frequencybeing greater than 0.2 Hz.
 11. The method according to claim 10 furthercomprising determining a magnitude of the component of the oscillationsin power lagging the phase of the oscillations in grid frequency by 90degrees.
 12. The method according to claim 1, wherein the at least oneelectrical connection comprises a plurality of electrical connectionsbetween the grid subsystem and the external electrical grid, and whereinthe oscillations in power conveyed corresponds to a summation of theoscillations in power conveyed on each of the plurality of electricalconnections.
 13. The method according to claim 1 wherein: the at leastone electrical connection comprises a plurality of electricalconnections between the grid subsystem and the external electrical grid;measuring oscillations in power conveyed comprises receiving a pluralityof quantities, each quantity corresponding to oscillations in powerconveyed on one of the plurality of electrical connections; anddetermining at least one phase relationship comprises determining aplurality of phase relationships between the oscillations in gridfrequency and the plurality of quantities to determine the components ofthe oscillations in power that contribute to oscillations in the gridfrequency in the external electrical grid.
 14. The method according toclaim 13 further comprising determining magnitudes of the components ofthe oscillations in power that are in quadrature with the oscillationsin grid frequency.
 15. A method of determining the contribution of agrid subsystem of a plurality of grid subsystems to oscillations in gridfrequency in an external electrical grid in an electrical power network,the method comprising: determining the contribution of a first gridsubsystem of the plurality of grid subsystems according to claim 1;determining the contribution of a second grid subsystem according toclaim 1, wherein a magnitude of the contribution of the first gridsubsystem is greater than a magnitude of the contribution of the secondgrid subsystem; and normalising to a maximum value between −1 and 1, themagnitude of the second grid subsystem to the magnitude of the firstgrid subsystem.
 16. The method according to claim 15 further comprisingdetermining that the second grid subsystem is contributing to theoscillations in grid frequency in the external electrical grid if thenormalised magnitude of the second grid subsystem is 0.5 or more.
 17. Acomputer program product comprising computer program code executable ona computer processor to carry out the method according to claim
 1. 18.An apparatus for determining contribution of a grid subsystem tooscillations in grid frequency in an external electrical grid in anelectrical power network, the apparatus comprising: a processorconfigured to: measure, via at least one phasor measurement unit,oscillations in grid frequency in the external electrical grid; receivea measurement of oscillations in power conveyed on at least oneelectrical connection between the grid subsystem and the externalelectrical grid; and determine the contribution of the grid subsystem tothe oscillations in the grid frequency in the external electrical gridby determining at least one phase relationship between the oscillationsin grid frequency and the oscillations in power conveyed to determine acomponent of the oscillations in power that contributes to theoscillations in grid frequency in the external electrical grid.
 19. Theapparatus according to claim 18 further comprising a first measurementunit in communications link with the processor and to the at least oneelectrical connection to the grid subsystem, the first measurement unitarranged to measure the oscillations in power conveyed and provide themto the processor.
 20. An electrical grid comprising the apparatusaccording to claim 18.